Heteroatom-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 128

7. 2 .1
β -Fragmentation, 1,2-H-Shift and H-Abstraction Reactions

Many O -centered radicals undergo facile -fragmentation. For example, acyl-

oxyl radicals which are intermediates in the electrolytic oxidation of acids
(Kolbe electrolysis), rapidly decompose into alkyl radicals and CO 2 [reaction
(1)]. The rate of these reactions is in the order of 10 9 s − 1 and increases with in-
creasing branching of the alkyl substituent, i.e., decreasing C

β

− CO 2 • bond energy

(Table 7.1).

R-C(O)O • → R • + CO 2 (1)

Similarly, the -fragmentation of tertiary alkoxyl radicals [reaction (2)] is a well-

known process. Interestingly, this unimolecular decay is speeded up in a polar
environment. For example, the decay of the tert -butoxyl radical into acetone and
a methyl radical proceeds in the gas phase at a rate of 10 3 s − 1 (for kinetic details
and quantum-mechanical calculations; see Fittschen et al. 2000), increases with
increasing solvent polarity (Walling and Wagner 1964), and in water it is faster
than 10 6 s − 1 (Gilbert et al. 1981; Table 7.2).

β

R 3 CO • → R • + R 2 C=O (2)

When different substituents can be cleaved by -scission the larger substituent

leaves preferentially [Rüchardt 1987; cf. reactions (3)

β

− (5)].

When alkoxyl radicals contain a hydrogen atom at the neighboring carbon, a
rapid 1,2-H-shift ( k 10 6 s − 1 or even faster) occurs in aqueous solution [reac-

tion (6); Berdnikov et al. 1972; Gilbert et al. 1976, 1977; Schuchmann and von
Sonntag 1981] with a KIE

≈

≈ 50 in the CH 3 CH 2 O • /CD 3 CD 2 O • system (Bonifacic

et al. 2003).

HR 2 CO •

• C(OH)R 2 + R 2 C=O (6)

→

This process must be water assisted (see also Elford and Roberts 1996), since
this reaction requires a high activation energy in the gas phase (Batt et al. 1981;